There are at least seventeen monoclonal antibodies currently approved in the United States for use as human therapeutics. Additionally, there are several hundred monoclonal antibodies in clinical trials and thousands in pre-clinical testing for treatment of various diseases or disorders including, e.g., transplant rejection, cancer, inflammatory diseases, sepsis, nephritis, Alzheimer's disease, allergies, diabetes, autoimmune diseases, arthritis, multiple sclerosis, and infectious diseases. The field of therapeutic monoclonal antibodies is positioned for rapid growth in the coming years. After vaccines, antibodies (or immunoglobulins, “Ig”) constitute the second most common type of biopharmaceutical agent being tested clinically (Stockwin, L. H. et al. Biochemical Society Transactions, 31:433-436, 2003).
Genetic engineering has contributed substantially to growth of the field of therapeutic monoclonal antibodies. The effectiveness of a potential therapeutic monoclonal antibody will often vary with modest changes to the protein sequence of the antibody. A single amino acid change in the variable region of a monoclonal antibody has the potential to alter the affinity with which the antibody binds the antigenic epitope, as well as antibody properties such as Kon rate or Koff rate. Such amino acid changes may determine the success or failure of a monoclonal antibody as a therapeutic. Similarly, modest changes in the amino acid sequence of the Fc region of a monoclonal antibody may yield profound changes in the antibody's effector function properties or the half-life of a protein to which the Fc region is operably linked.
The Fc region of an antibody (i.e., the carboxy-terminal ends of the heavy chains of an antibody spanning domains CH2, CH3 and a portion of the hinge region (see FIG. 1)), is limited in variability and is involved in effecting the physiological roles played by the antibody. The effector functions attributable to the Fc region of an antibody vary with the class and subclass of antibody and include (i) binding of the antibody via the Fc region to a specific Fc receptor (“FcR”) on a cell which triggers various biological responses including, e.g., phagocytosis and destruction of antibody-coated particles, clearance of immune complexes, release of inflammatory mediators, placental transfer of the antibody and control of immunoglobulin production, (ii) complement-dependent cytotoxicity (“CDC”) in which the Fc region binds the Clq component of complement and thereby initiates the classical pathway of complement activation which leads to lysis of the target, (iii) antibody dependent cell-mediated cytotoxicity (“ADCC”) in which certain human immune system cells, e.g., phagocytes and NK cells, via an Fcγ receptor, bind to the Fc region of an antibody via specific antibody-binding receptors on the immune cells and subsequently signal destruction of the entity to which the antibody is bound, and, (iv) binding to mast cells, basophils, and eosinophils. The affinity with which an Fc region can bind a particular FcR (e.g., FcRn), or the level with which an Fc region can mediate CDC or ADCC activity are important factors for determining the efficacy and half-life of therapeutic proteins, particularly monoclonal antibodies.
Particularized modification of amino acids in the Fc region of human IgG is an active area of study yielding structure-function relationship information relevant to development of therapeutic proteins, particularly monoclonal antibodies (see, e.g., U.S. Pat. No. 6,165,745 and PCT Publication No. WO2004/035752 regarding alteration of serum half-life of a polypeptide operably linked to an Fc region and U.S. Pat. No. 6,737,056 and PCT Publication No. WO2004/029207 regarding alteration of an effector function of a monoclonal antibody comprising a modified Fc region).
The development of novel therapeutic proteins, particularly monoclonal antibodies, would benefit from the ability to rationally design an Fc region with particular amino acid modifications that confer a desired beneficial property upon the antibody of interest. All monoclonal antibodies would not be expected to be improved as a therapeutic due to the same particular amino acid modification in the Fc region. A therapeutic monoclonal antibody that binds one target antigen may benefit from an increase in a particular effector function while a different therapeutic monoclonal antibody that binds a different target antigen may benefit from an increase in a different effector function, or even a decrease. One therapeutic monoclonal antibody may benefit from the ability to bind a particular Fc receptor with greater affinity while another antibody may be improved as a therapeutic by binding that Fc receptor at a lower affinity and therefore being cleared from the body at a faster rate. Furthermore, a particular Fc region amino acid modification or substitution and resulting effect that would benefit a therapeutic antibody may depend upon the antigenic target to which the antibody binds and/or the disease or disorder to be ameliorated by the antibody.
Methods and compositions that alter particular effector functions associated with the Fc region of an antibody are necessary to improve the properties of existing therapeutic antibodies as well as to generate novel therapeutic antibodies with desired properties. Monoclonal antibodies with variant Fc regions may be used to treat various diseases or disorders including, e.g., inflammatory disorders, cancer, autoimmune disorders, cell-signalling disorders and infectious diseases. Additionally, methods and compositions that alter the serum half-life of a therapeutic protein, either increasing the half-life and thereby allowing for fewer doses or decreasing the half-life and thereby allowing for more rapid clearance from the body, would benefit the generation of therapeutic antibodies as well as other therapeutic proteins.
What is needed in order to improve the efficacy of a therapeutic protein, particularly a monoclonal antibody, are variant Fc regions with improved properties.